extracellular environment, phospholipid bilayer, ion channels, transport proteins, cellular osmosis, calcium
Maintenance of proper cell volume is critical for cell survival and proliferation; consequently, animal cells routinely control their volume to maintain homeostasis. For instance, biological processes such as the response to ischemia (insufficient blood flow), lymphocyte activation, and apoptosis (programmed cell death) are associated with changes in cell volume. It has been well established that osmotic swelling associated with a hypotonic medium is followed by a regulated volume decrease (RVD) resulting from the efflux of specific solutes. However, the signaling mechanisms of RVD are ill defined. The effect of calcium on RVD was studied because this ion is a common messenger that plays a key role in the maintenance of a stable intracellular environment. American alligator (Alligator mississippiensis) red blood cells’ (RBCs) volume was measured electronically with a Coulter Counter® following hypotonic shock (0.5x Ringer). Studies using different extracellular concentrations of calcium, obtained with the chelator ethylene glycol bis(2-aminoethyl ether)-N,N,N'N'-tetraacetic acid (EGTA) (http://www.stanford.edu/~cpatton/maxc.html), showed that low extracellular calcium concentrations inhibited RVD. However, addition of the calcium ionophore A23187 in a normal Ringer caused a significant inhibition of RVD. Interestingly, substitution of the impermeant cation N-methyl-D-glucamine (NMDG) for sodium in the presence of A23187 also inhibited RVD, indicating that cell swelling was not dependent on sodium influx. Additional studies were also conducted to determine the mechanism(s) by which calcium ions cross the plasma membrane. A significant inhibition of RVD occurred in the presence of gadolinium, a blocker of stretch-activated channels. Other experiments with hexokinase (an ATP scavenger) suggested that calcium influx may occur via a purinoreceptor (P2 receptor). In conclusion, RVD by alligator RBCs depends on calcium influx via a stretch-activated channel and/or a P2 receptor.
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